Weeds can quickly run out of the valuable nutrients in the soil which are necessary for the growth of crops and other target plants. At present, there are many types of herbicides for weeds control, among which is a particularly popular herbicide, glyphosate. Glyphosate-resistant crops have been developed, such as corn, soybean, cotton, beet, wheat, and rice, and the like. Thus, it is possible to spray glyphosate in the fields planted with glyphosate-resistant crops to control weeds without significant damage to the crops.
Glyphosate has been widely used all over the world for more than 20 years, resulting in the overdependence on the technology of glyphosate and glyphosate-tolerant crops. In addition, high selection pressure has been forced to the naturally more glyphosate-tolerant plants among the wild weed species or the plants which have developed resistance to glyphosate activity. It has reported that a few weeds have developed resistance to glyphosate, including broad-leaved weeds and gramineous weeds, such as Swiss ryegrass, Lolium multiflorum, Eleusine indica, Ambrosia artemisiifolia, Conyza canadensis, Conyza bonariensis and Plantago lanceolata. In addition, the weeds which are not the agricultural problem before the widespread use of glyphosate-tolerant crops also gradually prevailed, and are difficult to be controlled with glyphosate-tolerant crops. These weeds mainly exist along with (but not only with) broad-leaved weeds which are difficult to be controlled, such as species from Amaranthus, Chenopodium, Taraxacum and Commelinaceae. 
In the area of glyphosate-resistant weeds or the weed species which are difficult to be controlled, growers can make up the weakness of the glyphosate through tank-mixing or using other herbicide which can control the omissive weeds. In most cases, a popular and effective tank-mixing partner used to control broad-leaved weeds is 2,4-dichlorophenoxyacetic acid (2,4-D). 2,4-D has been used to control broad-spectrum broad-leaved weeds more than 65 years under agriculture and non-crop conditions, and is still one of the most widely used herbicides in the world. The limit for further use of 2, 4-D is that its selectivity in dicotyledonous plants (such as soybeans or cotton) is very low. Therefore, 2,4-D is generally not used on (and generally not close to) sensitive dicotyledonous plants. In addition, the use of 2,4-D on gramineous crops is limited to a certain extent by the properties of the potential crop damage. The combination of 2,4-D and glyphosate has already been used to provide a stronger sterilization process before planting the no-till soybeans and cotton. However, due to the sensitivity of these dicotyledonous species to 2,4-D, these sterilization processes must be carried out 14 to 30 days before planting.
Same as MCPA, 2-methyl-4-chloropropionic acid and 2,4-D propionic acid, 2,4-De is also a phenoxy alkanoic acid herbicide. 2,4-D is used to selectively control broad-leaved weeds in many monocotyledonous crops such as corn, wheat and rice, without serious damage to the target crops. 2,4-D is a synthetic auxin derivative of which the function is to disorder the normal cytohormone homeostasis and to hinder the balance of controlled growth.
2,4-D shows different levels of selectivity on certain plants (for example, dicotyledonous plants are more sensitive than gramineous plants). Different 2,4-D metabolisms in different plants are one explanation for the different levels of selectivity. Plants usually metabolize 2, 4-D slowly. Thus, different activities of targeted points are more likely to explain different responses to 2, 4-D of plants. Plant metabolism of 2, 4-D is usually achieved through two steps of metabolism, i.e. the conjugation with amino acids or glucose following the hydroxylation in general.
As time goes on, the microbial populations have gradually developed effective, alternative pathways to degrade this particular foreign substance, which result in the complete mineralization of 2,4-D. Continuous application of herbicides on microbes can be used to select the microorganisms which use herbicides as carbon sources so as to make a competitive advantage in the soil. For this reason, 2,4-D was currently formulated with a relatively short soil half-life period and without obvious legacy effect on the subsequent crops, which promotes the application of 2, 4-D herbicide.
Ralstonia eutropha is one organism of which the ability for degrading 2,4-D has been widely studied. The gene encoding the enzyme in the first enzymatic step of mineralization pathway is tfdA. TfdA catalyzes the conversion of 2,4-D acid into dichlorophenol (DCP) through α-oxoglutarate-dependent dioxygenase reaction. DCP hardly has herbicide activity compared with 2,4-D. TfdA is used to indroduce 2,4-D resistance into dicotyledonous plants which are usually sensitive to 2, 4-D (such as cotton and tobacco) in transgenic plants.
A number of tfdA type genes have been identified which encode proteins capable of degrading 2,4-D in the environment. Many homologs are similar with tfdA (amino acid identity >85%) and have similar enzyme activity with tfdA. However, not all proteins having structures such as TauD which have the function of degrading 2,4-D, and a large number of homologs have significantly lower identity (25-50%) with tfdA while contain characteristic residues associated with α-oxoglutarate-dependent dioxygenase Fe2+ dioxygenases. Therefore, the substrate specificities of these different dioxygenases are indefinite. A unique instance which has low homology (28% amino acid identity) with tfdA is rdpA from Sphingobium herbicidovorans. It has been shown that this enzyme catalyzes the first step in the mineralization of (R)-2,4-D propionic acid (and other (R)-phenoxy propionic acids) and 2, 4-D (phenoxyacetic acid).
With the emergence of glyphosate-resistant weeds and the expanded application of 2,4-D herbicide, it is necessary to introduce 2,4-D resistance into the target plants sensitive to 2,4-D. At present, no reports have been found about the expression levels of herbicide-resistant protein 24DT21 in plants and their herbicide tolerance.